1. Field of the Invention
The present invention relates to an optical glass element, more specifically a micro-optical glass element such as an optical-communication-use coupling lens, an optical pickup lens and an endoscope-use lens, and a manufacturing method for such a lens.
2. Description of the Related Art
In recent years, in the field of compact cameras, VTR cameras, optical pickup lenses used for DVDs and the like, micro-optical glass elements, molded into predetermined shapes, have been utilized in order to achieve a small size, high precision and environment resistance. In order to manufacture such a micro-optical glass element, as shown in FIG. 7, a method in which, after molten glass 53 has been directly dropped onto a heated lower mold 51, this is pressed between the lower mold 51 and an upper mold 52 has been often used. In particular, during the press-molding process, a border area 56 between a molding face 54 belonging to the lower mold 51 and its circumferential face 55 is generally made in contact with glass 53. In general, the optical glass element has an edge portion 57 around a lens portion 58, which is formed from the constraint in the manufacturing process.
There arises a problem that in the case when a lens is manufactured through this method, however, chips and cracks tend to occur on the border between the lens portion and the edge portion. Presumably, this problem is caused by the fact that, since the thickness of the edge portion is generally thinner than the lens portion, there is a difference in shrinkage of glass between the lens portion and the edge portion in a glass cooling process upon press-forming the lens. More specifically, since the amount of shrinkage of the edge portion is smaller than the amount of shrinkage of the lens portion, the pressure to be received by the edge portion increases when a pressure is continuously applied to the glass in a manner so as to allow the mold to follow the shrinkage of glass in the lens portion. As a result, a stress is concentrated on the border between the lens portion and the edge portion to cause chips and cracks. This problem becomes more conspicuous in the case when a lens in which the thickness of the edge portion is extremely smaller than the center thickness of the lens, that is, a so-called convex lens, is manufactured. In contrast, in the case when a pressure is applied to the glass in a manner so as to allow the mold to follow the shrinkage of glass in the edge portion, the molding surface is not preferably transferred onto the lens portion.
In order to solve these problems, a technique in which a chamfered portion is formed on the border portion between the processing curved portion and plane portion of the mold (for example, see Japanese Patent Application Publication No. 3-52417 (Japanese Patent Laid-Open No. 63-151632)) has been proposed. In this technique, however, since time-consuming tasks including the chamfering process added to the mold-forming process are required and since the grindstone is more susceptible to abrasion depending on the amount of chamfered portion, there is an increase in the manufacturing costs of the mold.
Another technique has been proposed in which the border portion between the processing curved portion and the plane portion is formed into an R-curved shape so as to prevent cracks in the lens (for example, see Japanese Patent Application Laid-Open No. 6-263462). However, this technique also causes problems in that time-consuming tasks are required in the mold-forming process, in that the manufacturing costs of the mold increase and in that it is impossible to simultaneously process the mold-processing face and the edge portion.